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Evaluation of Automatic Hypocenter Determination in the JMA Unified Tamaribuchi Earth, Planets and Space (2018) 70:141 https://doi.org/10.1186/s40623-018-0915-4 FULL PAPER Open Access Evaluation of automatic hypocenter determination in the JMA unifed catalog Koji Tamaribuchi* Abstract The Japan Meteorological Agency (JMA) unifed seismic catalog has been widely used for research and disaster pre- vention purposes for more than 20 years. Since the introduction in April 2016 of an improved method of automatic hypocenter determinations (PF method), the number of detected earthquakes has almost doubled due to a decrease in the completeness magnitude around the Tohoku region, where seismicity has been very active in the aftermath of the 2011 Tohoku earthquake. Automatically processed hypocenters of small events, accepted without manual modifcation, now make up approximately 70% of new events in the JMA unifed catalog. In this paper, we show that the introduction of automated processing did not systematically bias the quality of the JMA unifed catalog. Approxi- mately 90% of automatically processed hypocenters were less than 1 km from their manually reviewed locations in inland and shallow areas. We also considered the use of automated event characterization in real-time monitoring of earthquake sequences using the example of the April 2016 Kumamoto earthquake sequence, when the PF method could have supplied the catalog with about 70,000 events in real time over the course of 2 months. We show that the PF method is capable of monitoring the migration or expansion of the hypocentral distribution and can support sta- tistical analyses such as variations of the b-value distribution. Further improvements in automatic hypocenter deter- mination will contribute to a better understanding of seismicity as well as rapid risk assessment, especially in cases of swarms and aftershocks. Keywords: JMA unifed seismic catalog, Automatic hypocenter determination, 2016 Kumamoto earthquake, Completeness magnitude Introduction Research Institute for Earth Science and Disaster Resil- A high-quality earthquake catalog, consistently com- ience (NIED), universities, and other institutes. Te JMA piled over a wide spatiotemporal range, is an important unifed catalog is published and routinely updated on the research tool to analyze the history of seismicity, under- web (JMA 2018) and is widely used for research, promo- stand earthquake mechanisms, and elucidate the under- tion of disaster prevention activities, and other purposes. ground structure of seismogenic regions. Te Japan Te 2011 Mw 9.0 Tohoku earthquake produced an Meteorological Agency (JMA) has recorded earthquake enormous number of aftershocks and induced earth- data in Japan for over 100 years (e.g., Schorlemmer quakes in and around Japan. Because all hypocenters et al. 2018). In October 1997, after the 1995 Kobe earth- required a visual check before being added to the catalog, quake, the JMA started to produce a consistent catalog publication of the catalog was considerably delayed. To of seismic events (hereafter the JMA unifed catalog) cope with this situation, the catalog was limited to events in cooperation with the Ministry of Education, Cul- within the Tohoku region of M ≥ 2 (JMA magnitude) ture, Sports, Science and Technology (MEXT), by using for inland locations and M ≥ 3 for ofshore areas. Nev- seismic waveforms recorded by the JMA, the National ertheless, it took over 2 years to determine over 250,000 hypocenters that occurred during 2011. In addition, the *Correspondence: ktamarib@mri‑jma.go.jp number of monthly events in the JMA unifed catalog Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, doubled after 2000 (Fig. 1) with the deployment of Hi-net Japan observation stations by NIED from 2000 to 2002 (Okada © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Tamaribuchi Earth, Planets and Space (2018) 70:141 Page 2 of 10 the automatically estimated hypocenter is cataloged All events M ≥ 1 when the inspector fnds that it was properly determined. M ≥ 2 Hypocenters that are not properly determined can still be cataloged after a simple review. With this change in pro- cedure, the magnitude limitation in the Tohoku earth- quake aftershock area was canceled at the end of March 2016. Enough time has elapsed since the change in the cata- log procedure to evaluate whether the new procedure caused a systematic bias in the quality of the JMA unifed catalog. Tis evaluation, reported in this paper, is impor- tant to ensure the continued reliability of the JMA unifed catalog for seismic research. In addition, it is important to evaluate the use of initial hypocenter determinations Year (before visual inspection) for real-time monitoring of Fig. 1 Monthly frequency of events in the JMA unifed catalog seismic activity, especially for the abundant aftershocks since 1990. Blue bars indicate the number of earthquakes. The red line indicates the number of stations. Automatic hypocenter of large events. In this paper, we use the M 7.3 Kuma- determinations using the PF method (Tamaribuchi et al. 2016) were moto earthquake sequence in April and May 2016 as a introduced in April 2016 case study. Methods Te PF method was described by Tamaribuchi et al. et al. 2004). Te number of detected earthquakes will (2016) in Japanese, so for convenience a description in further increase with the deployment of seafoor seis- English is presented in Additional fle 1. Here we will mic observation stations such as those in DONET1 and focus on the conventional group trigger method (GT DONET2, developed by the Japan Agency for Marine- method), previously used for the JMA catalog since its Earth Science and Technology (JAMSTEC) and oper- beginning in October 1997, and describe its problems ated by NIED, and S-net, developed and operated by and their solutions by using the PF method. NIED. To aid prompt processing of large amounts of data Figure 2 presents fow diagrams for the GT and PF from these high-sensitivity seismic networks, the Earth- methods. In the GT method, the ratio of the long-term quake Research Committee (ERC) of the Headquarters average (LTA) to short-term average (STA) amplitude, for Earthquake Research Promotion of Japan established or STA/LTA (e.g., Allen 1978), is calculated for seismic three policies: (1) maintaining the present detection lim- waveforms at each observation station, and when the its of the catalog, (2) cataloging all detected earthquakes, ratio exceeds a threshold value, the observation station is and (3) categorizing events based on a quality control regarded as triggered. When a certain number of stations procedure (ERC 2014). Based on these policies, in April are triggered within a certain time range within a prede- 2016 the JMA adopted the Phase combination Forward fned group of stations (for example, four or more sta- search method (PF method; Tamaribuchi et al. 2016) for tions are triggered within 120 s), it is defned as an event automatic hypocenter determination to streamline pro- detection. At this point, the trigger time at each station is duction of the JMA unifed catalog. Te PF method can taken as the P-phase arrival time and a hypocenter is cal- detect simultaneously occurring earthquakes by using a culated from the P-phases (frst-round hypocenter calcu- Bayesian estimation approach. Details of this method are lation). On the basis of the theoretical travel times from described in “Methods” section and Additional fle 1. this initial hypocenter estimation, P-wave and S-wave Automatically processed hypocenters are added to arrivals are automatically picked again and a revised the JMA unifed catalog after quality control by visual hypocenter is calculated from the P- and S-phases (sec- inspection and the assignment of a status fag to each ond-round hypocenter calculation). Phase-picking is event that characterizes its magnitude and accuracy. Te repeated, based on travel times from successively revised procedure for large events did not change in April 2016; hypocenters, until the hypocenters converge to a stable that is, all events greater than the threshold magnitude location. An advantage of the GT method is that it is (such as Mth 1.7 in shallow inland areas and increasing = possible to exclude local noise that afects a single sta- to Mth 3.5 with distance from land; see also JMA 2018 = tion. Also, even when the hypocenter does not converge, in detail) are visually inspected before inclusion in the an event detection can alert the operators to perform a catalog. For events smaller than the threshold magnitude, visual inspection and correctly determine the hypocenter. Tamaribuchi Earth, Planets and Space (2018) 70:141 Page 3 of 10 Because the GT method assumes that noise rarely a GT method b PF method occurs at the same time as earthquakes, it can ignore isolated local noise. However, when an earthquake and Seismic Seismic noise coincide, or when two or more earthquakes occur waveform waveform simultaneously, the method leads to incorrect locations in the frst-round hypocenter calculation. Also, when many earthquakes occur, such as during aftershocks and Trigger by STA/LTA Extract feature swarms, the LTA value is high, so the detection capabil- (P-wave
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